Catalytic dehydrogenation of hydrocarbons



United States Patent 3,248,451 CATALYTIC DEHYDROGENATIGN 0F HYDROCARBONS Lawrence 1. Hughes, Hitchcock, Tex., assignor to Monsanto Company, a corporation of Delaware No Drawing. Filed Apr. 19, 1963, Ser. No. 274,296 4 Claims. (Cl. 260-683.?!)

The present invention relates to a process for the catalytic dehydrogenation of aliphatic hydrocarbons. More particularly, it relates to an improved process for the catalytic dehydrogenation of long-chain saturated hydrocarbons containing from 9 to 16 carbon atoms to the corresponding unsaturated hydrocarbons containing the same number of carbon atoms tothe molecule. It is well known that saturated aliphatic hydrocarbons can be converted to unsaturated hydrocarbons by catalytic dehydrogenation. A wide variety of catalytic sys terns and processes have been developed in the art for the dehydrogenation of n-paraffins to produce the corresponding olefins. A classic example in point is the conversion of n-butane to butenes. For the most part, the prior art processes are directed to the catalytic dehydrogenation of light, normally-gaseous parafiins, that is, C C and C hydrocarbons, employing so-called aluminachromia type catalysts. These and other generally known catalysts, however, are not suitable for the catalytic dehydrogenation of the higher normal or straightchain parafiins such as hexane and particularly longchain n-parafiins, that is, n-parafiins having from 9 to 16 carbon atoms. When employed with such hydrocarbons they cause formation of aromatic hydrocarbons and relatively smaller amounts of olefines. Another problem arises from the fact that long-chain high boiling molecules are more easily cracked than the short-chain, lowmolecular weight molecules. Under the normal dehydrogenatin-g conditions, therefore, a major concomitant reaction in the catalytic dehydrogenation of long-chain n-parafiins is cracking or splitting of carbon-carbon bonds to form two or more lighter hydrocarbons one of which is usually less saturated than the other. This produces an unsaturated hydrocarbon difierent from that produced by simple dehydrogenation and an undesirable light paraflin. The product thus becomes a heterogeneous one from .which the desired olefin can be recovered only in poor ultimate yields by a series of purification operations which can be tedious and expensive.

Accordingly, it is an object of the present invention to provide an improved process for the catalytic dehydrogenation of long-chain saturated hydrocarbons to the corresponding unsaturated hydrocarbons containing the same number of carbon atoms to the molecule which overcomes the disadvantages of the prior art.

It is a further object of the invention to provide an improved process for the catalytic dehydrogenation of normal parafiins containing from 9 to 16 carbon atoms to the corresponding monoolefins.

These and other objects and advantages of the invention which will be apparent from the following description thereof are obtained by contacting at elevated temperatures a normal paraffin containing from 9 to 16 carbon atoms in the vapor phase with a catalyst comprising cobalt molybdate supported on alumina or activated alumina. In the specific embodiment of the invention, the long-chain normal paraffins are converted to olefins by diluting them with up to ten volumes of inert gas per volume of hydrocarbon and then contacting the mixed gases with cobalt molybdate supported on alumina or activated alumina at a temperature within the range from about 350 to about 650 C. and particularly from about 475 to about 575 C. at contact times varying from about 0.5 to about 5.0 sec. It is surprising that 3,248,451 Patented Apr. 26, 1966 the cobalt molybdate catalyst can be used to convert long-chain n-paraflins to the corresponding olefins in view of the prior art teachings that the higher boiling members of the parafiinic hydrocarbons such as heptanes, hexanes, octanes and the higher members of the homologous series will tend to form unsaturated ring compounds such as aromatics when catalytically dehydrogenated.

The invention is illustrated in the following examples which, however, are not to be construed as limiting it in any manner whatsoever.

Example 1 The dehydrogenation reactoremployed was a glass tube approximately 1 in. in outside diameter and 23 in. long fitted with a 6 mm. O.D. thermowell. It was Wrapped with Nichrome ribbon for heating and covered with hightemperature pipe insulation.

The reactor was packed with a commercially available cobalt molybdate catalyst supported on alumina constituted of 3.2% cobalt oxide and 13.0% molybdenum oxide, the remainder of the composition being alumina. The reactor was heated to reaction temperature while a stream of dry nitrogen was passed through it. Highpurity grade (99+)% n-dodecane was vaporized and mixed with steam in a volume ratio of two parts of steam to one of hydrocarbon. The mixture was passed through a short preheater maintained at about 250 C. and then through the catalyst bed in the reactor after the temperature had become stabilized at the desired level. Pressure was maintained at essentially atmospheric throughout all the runs.

The efiluent gas was conducted through a cold water condenser. All uncondensed gases were vented. The contents of the condenser-receiver were analyzed by means of gas chromatography. Based on the analysis, the ultimate yield of dodecene was calculated assuming 100% recovery. Results obtained in several runs are presented below together with the conditions under which they were obtained.

The experiment of Example 1 was repeated using another commeroial sample of cobalt molybdate 0n alumina comprising 2.1% cobalt oxide and 8.0% molybdenum trioxide in the form of x A" tablets. in one run over the temperature range from 520 to 590 C. with an average temperature of about 550 C. and a contact time of 2 seconds, approximately 6.0% of the n-dodecane fed was converted to dodecene per pass and an ultimate yield of dodecene of 83.4% was obtained. In a second similar run at the same contact time but in a somewhat lower temperature range from 515 to 545 C. and an average temperature of 530 C., the conversion of n-dodecane lto dodecene was about 4.1% While the ulti-.

mate yield of this olefin was 85.5%.

Example 3 Following essentially the same procedure and using the same catalyst as in Example 1, an alkane fraction or kerosene cut from which the isoparaflins had been removed consisting of n-parafiins having from 10 to 14 carbon atoms was dehydrogenated in a tubular stainless steel reactor /2 in. in outside diameter and about 12 in. long. Several runs were made at reduced pressure (4.0 in. Hg absolute) and at a temperature of about 530 C. In each of the runs, the hydrocarbon feed was diluted with nitrogen to provide a nitrogen-to-hydrocarbon ratio of 2 to 1 and flow rates were regulated to give a contact time of about 0.3 second in the first run and about 1.5 seconds in the other two runs. The conversion to alkenes containing 10 to 14 carbon atoms in each case was about 10% while the ultimate yields of olefins were 91%, 86% and 94%, respectively.

Variations in procedure and reaction conditions from those given in the examples may be made without departing from the scope of the invention. For example, the dehydrogenation process of the invention is applicable generally to straight-chain saturated aliphatic hydrocarbons having from 9 to 16 carbon atoms such as n-nonane, n-decane, n-undecane, n-tridecane, n-tetradecane, n-pentadecane and n-hexadecane. These compounds may be reacted individually or mixtures of the compounds may be employed as the starting material in the dehydrogenation process. If desired, the dehydrogenation may be effected in the presence of relatively inert gases as diluents. Suitable diluents in addition to the steam and nitrogen exemplified include stable hydrocarbons such as methane, hydrogen, argon, and the like. When a diluent is employed any ratio of diluent to hydrocarbon up to about 10:1 maybe employed. Preferred diluent-to-hydrocarbon ratios, particularly when steam is used, are those in the range from 2:1 to 4:1.

The catalyst employed herein described as cobalt molybdate is a mixture of cobaltous oxide (C) and molybdenum oxide, generally molybdic trioxide (M00 supported upon alumina. Many varieties of such catalysts are commercially available. Generally, for the purposes of this invention, the cobalt molybdate contains from about 1% to about 5% by Weight of cobaltous oxide and from about 5% to about 20% by weight of molybdic trioxide with the remainder of the catalyst composition being alumina. Preferred catalyst compositions contain from about 2% to about 4% by weight of cobaltous oxide and from about 7% to about 13% by weight of molybdic trioxide on alumina. Preferably, gamma alumina is employed as the catalyst support. The catalyst may be used in any suitable solid form such as powder, granules, tablets and the like.

Withcontinued use, the catalyst may become deactivated by deposition of carbon thereon. When this occurs, it can be readily reactivated or regenerated by burning olf the carbon in a stream of oxygen or air suitably diluted if necessary with inert gas.

The vapors of the long-chain parafiin hydrocarbon to be dehydrogenated can be contacted with the catalyst at any temperature within the range from 350 to 650 C. Preferably, temperatures from about 475 to about 575 C. are employed depending upon the particular hydrocarbon feed material. Ordinarily, atmospheric pressure is used but the process can be conducted advantageously under reduced pressures. Superatmospheric pressures, while they can be used, are usually to be avoided because at high pressures the equilibrium is shifted to favor the paraflin.

Contact time is not critical in that times of contact and temperature are interchangeable over a fairly wide range and vary likewise with the particular catalyst composition employed. Generally, contact times employed may vary from about 0.1 to about 10 seconds. Preferred contact times of 2 to 4 seconds are used in conjunction with the preferred temperatures.

Any of a number of materials can be employed for the construction of the reactor. Essentially the same results are obtained with reactor tubes fabricated from copper, glass, and stainless steels, for example.

What is claimed is:

1. An improved process for the dehydrogenation of long-chain n-parafi'lns to produce long-chain n-olefins having a corresponding number of carbon atoms in the molecule which comprises contacting n-parafiins containing from 9 to 16 carbon atoms in the vapor phase with a catalyst consisting essentially of cobalt molybdate supported on alumina at temperatures from about 350 C. to about 650 C. and at contact times from about 0.1 to about 10 seconds.

2. An improved process for the dehydrogenation of long-chain n-parafiins to produce long-chain n-olefins having a corresponding number of carbon atoms in the molecule which comprises contacting n-parafiins containing from 9 to 16 carbon atoms in the vapor phase and diluted with a relatively inert gas in volume proportions up to 10 parts of inert gas per part of paraffin with a catalyst consisting essentially of cobalt molybdate supported on alumina, said catalyst containing from about 1% to about 5% by weight of cobaltous oxide and from about 5% to about 20% by weight of molybdic trioxide, at temperatures from about 350 C. to about 650 C. and at contact times from about 0.1 to about 10 seconds.

3. An improved process for the dehydrogenation of long-chain n-paraflins to produce long-chain n-olefins having a corresponding number of carbon atoms in the molecule which comprises contacting n-paraffins containing from 9 to 16 carbon atoms in the vapor phase and diluted with an inert gas to provide a diluent-to-hydrocarbon volume ratio from about 2:1 to about 4:1 with a catalyst consisting essentially of cobalt molybdate supported on alumina, said catalyst containing from about 1% to about 5% by weight of cobaltous oxide and from about 5% to about 20% by weight of molybdic trioxide at temperatures from about 475 C. to about 575 C. and at contact times from about 0.1 to about 10 seconds.

4. An improved process for the dehydrogenation of long-chain n-parafiins to produce long-chain n-olefins having a corresponding number of carbon atoms in the molecule which comprises contacting n-paraifins containing from 9 to 16 carbon atoms in the vapor phase and diluted with an amount of steam to provide a steam-to-hydrocarbon volume ratio of about 2:1 to about 4:1 with a catalyst consisting essentially of cobalt molybdate supported on alumina, said catalyst containing from about 2% to about 4% by weight of cobaltous oxide and from about 7% to about 13% by weight of molybdic trioxide, at temperatures from about 474 C. to about 575 C. and at contact times from about 2 to about 4 seconds.

References Cited by the Examiner UNITED STATES PATENTS 2,867,581 1/1959 Nahin 208136 3,023,254 2/1962 Othmer et al. 260683.3 3,159,688 12/1964 Jennings et al. 260- 680 DELBERT E. GANTZ, Primary Examiner.

ALPHONSO D. SULLIVAN, Examiner.

S. P. JONES, Assistant Examiner. 

1. AN IMPROVED PROCESS FOR THE DEHYDROGENATION OF LONG-CHAIN N-PARAFFINS TO PRODUCE LONG-CHAIN N-OLEFINS HAVING A CORRESPONDING NUMBER OF CARBON ATOMS IN THE MOLECULE WHICH COMPRISES CONTACTING N-PARAFFINS CONTAINING FROM 9 TO 16 CARBON ATOMS IN THE VAPOR PHASE WITH A CATALYST CONSISTING ESSENTIALLY OF COBALT MOLYBDATE SUPPORTED ON ALUMINA AT TEMPERATURES FROM ABOUT 350*C. TO ABOUT 650*C. AND AT CONTACT TIMES FROM ABOUT 0.1 TO ABOUT 10 SECONDS. 